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Abstract:

The subject matter described herein includes methods, systems, and
computer program products for measuring the density of a material.
According to one aspect, a material property gauge includes a nuclear
density gauge for measuring the density of a material. A radiation source
adapted to emit radiation into a material and a radiation detector
operable to produce a signal representing the detected radiation. A first
material property calculation function may calculate a value associated
with the density of the material based upon the signal produced by the
radiation detector. The material property gauge includes an
electromagnetic moisture property gauge that determines a moisture
property of the material. An electromagnetic field generator may generate
an electromagnetic field where the electromagnetic field sweeps through
one or more frequencies and penetrates into the material. An
electromagnetic sensor may determine a frequency response of the material
to the electromagnetic field across the several frequencies.

Claims:

1. A material property gauge for determining a property of a material,
the material property gauge comprising: (a) a nuclear density gauge for
measuring the density of a material, the nuclear density gauge
comprising: a radiation source adapted to emit radiation into the
material; a radiation detector operable to produce a signal representing
the detected radiation; and a first material property calculation
function configured to calculate a value associated with the density of
the material based upon the signal produced by the radiation detector;
and (b) an electromagnetic moisture property gauge for determining a
moisture property of the material, the electromagnetic moisture property
gauge comprising: an electromagnetic field generator configured to
generate an electromagnetic field including sweeping through one or more
frequencies and penetrating into a material, wherein the material
includes at least one of a pavement material, aggregate base material,
concrete, and a soil material; an electromagnetic sensor configured to
determine a frequency response of the material to the electromagnetic
field across the one or more frequencies; and a second material property
calculation function configured to correlate the frequency response to a
moisture property of the material and to calculate a value representing
the moisture property; and (c) a third material property calculation
function for determining a material property of the material based on the
value associated with the density of the material and the value
representing the moisture property of the material.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser.
No. 13/089,196, filed Apr. 18, 2011, which is a continuation of U.S.
patent application Ser. No. 12/910,745 (now U.S. Pat. No. 7,928,360),
filed Oct. 22, 2010, which is a continuation of U.S. patent application
Ser. No. 12/534,739 (now U.S. Pat. No. 7,820,960), filed Aug. 3, 2009,
which is a continuation of U.S. patent application Ser. No. 11/512,732
(now U.S. Pat. No. 7,569,810), filed Aug. 30, 2006, which claims the
benefit of U.S. Provisional Patent Application Ser. No. 60/712,754, filed
Aug. 30, 2005, and U.S. Provisional Patent Application Ser. No.
60/719,071, filed Sep. 21, 2005, the disclosures of which are
incorporated by reference herein in their entireties. The disclosure of
U.S. patent application Ser. No. 11/513,334, filed Aug. 30, 2006, is
incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The subject matter described herein relates to measuring material
properties. More particularly, the subject matter described herein
relates to methods, systems, and computer program products for measuring
the density of a material including a non-nuclear moisture property
detector.

BACKGROUND

[0003] In construction engineering, some of the most important properties
of interest are volumetric and mechanistic properties of a bulk soil
mass. In particular, there are procedures in construction engineering
practice that relate total volume Vt, mass of water Mw, and
mass of dry solids Ms to the performance of a structure built on a soils
foundation. Thus, the measurements of these properties are important for
construction engineering.

[0004] Material density and moisture content are other important material
properties used for design, quality control, and quality assurance
purposes in the construction industry. Some exemplary techniques for
measuring the density and moisture content of soils include nuclear, sand
cone, and drive cone, as described by the American Society of Testing and
Materials (ASTM) standards D-2922, D-3017, and D-1556, and the American
Association of State Highway and Transportation Officials (AASHTO)
standards T-238, T-239, T-191, and T-204. The nuclear measurement
technique is non-destructive and calculates both the density and the
moisture content in a matter of minutes. The sand cone and drive cone
measurement techniques require the moisture content test of ASTM standard
D-2216, which involves a time consuming evaporation process. The moisture
content test involves heating a sample to 110° C. for at least 24
hours.

[0005] For road construction, there is an optimum water or moisture
content that allows for obtaining a maximum density. An exemplary density
test is described in ASTM standard D-698, wherein a field sample is
prepared with different water contents, and compacted with like energy
efforts. Hence, each sample has different water content, but the same
compaction effort. The densities are then measured gravimetrically in the
laboratory. The moisture content with the highest density is deemed the
optimum condition and selected as the field target. Summarily, the
objective of material compaction is the improvement of material
properties for engineering purposes. Some exemplary improvements include
reduced settling, improved strength and stability, improved bearing
capacity of sub grades, and controlling of undesirable volume changes
such as swelling and shrinkage.

[0006] In the road paving and construction industry, portable nuclear
density gauges are used for measuring the density of asphalt pavement and
soils. Often, an asphalt paving material is applied on a new foundation
of compacted soil and aggregate materials. The density and moisture
content of the soil and aggregate materials should meet certain
specifications. Therefore, nuclear gauges have been designed to measure
the density of the asphalt pavement and soils.

[0007] Nuclear density gauges typically include a source of gamma
radiation which directs gamma radiation into the sample material. A
radiation detector may be located adjacent to the surface of the sample
material for detecting radiation scattered back to the surface. From this
detector reading, the density of the sample material can be determined.

[0008] These nuclear gauges are generally designed to operate either in a
backscatter mode or in both a backscatter mode and a transmission mode.
In gauges capable of transmission mode, the radiation source is
vertically moveable from a backscatter position, where it resides within
the gauge housing, to a series of transmission positions, where it is
inserted into holes or bores in the sample material to selectable depths.

[0009] Nuclear gauges capable of measuring the density of sample materials
have been developed by the assignee of the present subject matter. For
example, nuclear gauges for measuring the density of sample materials are
disclosed in U.S. Pat. Nos. 4,641,030; 4,701,868; and 6,310,936, all of
which are incorporated herein by reference in their entirety. The gauges
described in these patents use a Cesium-137 (Cs-137) source of gamma
radiation for density measurements, and Americium Beryllium (AmBe)
neutron sources for moisture measurements. Paving material may be exposed
to the gamma radiation produced by the Cs-137 source. Gamma radiation is
Compton scattered by the paving material and detected by Geiger-Mueller
tubes positioned to form at least one geometrically differing
source-to-detector relationships. The density of the paving material is
calculated based upon the gamma radiation counts detected by the
respective detectors.

[0010] One difficulty to the use of nuclear density gauge is the use of a
radioactive source and the associated regulations imposed by the U.S.
Nuclear Regulatory Commission (NRC). The requirements for meeting NRC
regulations are largely dependent on the quantity of radioactive source
material used in a gauge. Thus, it is desirable to provide a nuclear
density gauge having a smaller quantity of radioactive source material in
order to reduce the requirements of the NRC for use of the gauge.

[0011] Another difficulty with nuclear gauges is the time required for
making a density measurement of material. Delays in obtaining density
measurements of soils during construction may delay or otherwise disturb
the construction process. Thus, it is desirable to provide a nuclear
density gauge operable to provide faster density measurements.

[0012] Accordingly, in light of the above described difficulties and needs
associated with nuclear density gauges, there exists a need for improved
methods, systems, and computer program products for measuring the density
of material.

SUMMARY

[0013] The subject matter described herein includes methods, systems, and
computer program products for measuring the density of a material.
According to one aspect, a material property gauge includes a nuclear
density gauge for measuring the density of a material. The nuclear
density gauge includes a radiation source adapted to emit radiation into
a material and a radiation detector operable to produce a signal
representing the detected radiation. A first material property
calculation function is configured to calculate a value associated with
the density of the material based upon the signal produced by the
radiation detector. The material property gauge further includes an
electromagnetic moisture property gauge configured to determine a
moisture property of the material. The electromagnetic moisture property
gauge includes an electromagnetic field generator configured to generate
an electromagnetic field where the electromagnetic field sweeps through
one or more frequencies and penetrates into the material. The material
includes at least one of a pavement material, aggregate base material,
concrete, and a soil material. An electromagnetic sensor is configured to
determine a frequency response of the material to the electromagnetic
field across the one or more frequencies. A second material property
calculation function is configured to correlate the frequency response to
a moisture property of the material and to calculate a value representing
the moisture property. The material property gauge further includes a
third material property calculation function configured to determine a
material property of the material based on the value associated with the
density of the material and the value representing the moisture property
of the material. As used herein, the terms "sample construction
material," "sample material," "construction material," and "material"
refer to any suitable material used in a construction process. Exemplary
sample construction materials include soil, asphalt, pavement, stone,
sub-base material, sub-grade material, cement, agricultural soils, batch
plants, concrete curing rate, concrete chloride inclusion, sodium
chloride content, concrete delamination, water content, water-cement
materials, alkali-silica, various soils, flexible asphalt, and any
combination thereof.

[0015] The foregoing summary, as well as the following detailed
description of various embodiments, is better understood when read in
conjunction with the appended drawings. For the purposes of illustration,
there is shown in the drawings exemplary embodiments; however, the
presently disclosed subject matter is not limited to the specific methods
and instrumentalities disclosed.

[0016] In the drawings:

[0017] FIG. 1A is a graph of a comparison of dielectric constants of clay
material and non-clay material over different frequencies;

[0018] FIG. 1B is a graph of dielectric constant dispersion of several
different types of clays;

[0019] FIG. 1C is a graph of dielectric dispersion of the conductivity and
dielectric constant of cohesive soil;

[0020]FIG. 2 is a vertical cross-sectional view of a nuclear density
gauge for measuring the density of material according to an embodiment of
the subject matter described herein;

[0021] FIG. 3 is a vertical cross-sectional view of the nuclear density
gauge shown in FIG. 2 configured in a transmission mode for measuring the
density of a sample material according to an embodiment of the subject
matter described herein;

[0022] FIG. 4 is a flow chart of an exemplary process by which the gauge
shown in FIGS. 2 and 3 may be initialized according to an embodiment of
the subject matter described herein;

[0023] FIG. 5 is a flow chart of an exemplary process for determining
detector counts within an energy window according to an embodiment of the
subject matter described herein;

[0024] FIG. 6 is a vertical cross-sectional view of the nuclear density
gauge shown in FIGS. 2 and 3 for measuring the density of asphalt layers
according to an embodiment of the subject matter described herein;

[0028] FIG. 10 is a graph of density measurements for various gamma-ray
energy bands as a glass thickness was varied;

[0029] FIG. 11 is a graph of the calibration curves for granite and
limestone mixes as determined from experimentation;

[0030] FIG. 12 is a flow chart of an exemplary process for density
measurements in a backscatter mode using the gauge shown in FIG. 6
according to an embodiment of the subject matter described herein; and

[0031]FIG. 13 is a flow chart of an exemplary process for density
measurements in a transmission mode using the gauge shown in FIG. 3
according to an embodiment of the subject matter described herein.

DETAILED DESCRIPTION

[0032] The subject matter described herein includes methods, systems, and
computer program products for measuring the density of a material and/or
various other material properties. In one embodiment, the methods,
systems, and computer program products described herein may determine the
radiation propagation properties of a material under test for measuring
the density of the material. According to one aspect, a nuclear density
gauge may include a radiation source for positioning in an interior of a
sample material, such as soil. The radiation source may emit radiation
from the interior of the sample material for detection by a radiation
detector. The radiation detector may produce a signal representing an
energy level of detected radiation. The nuclear density gauge may also
include a material property calculation function configured to calculate
a value associated with the density of the sample material based upon the
signals produced by the radiation detector.

[0033] In another embodiment, the methods, systems, and computer program
products described herein may determine the radiation propagation and
moisture properties of a material under test for measuring the density of
the material. The material may be a construction related material such as
soil or asphalt or concrete. In one aspect, a material property gauge may
include a radiation source positioned for emitting radiation into a
material under test. A radiation detector may detect radiation from the
material and produce a signal representing the detected radiation. A
moisture property detector may determine a moisture property of the
material and produce a signal representing the moisture property. The
material property gauge may include a material property calculation
function configured to calculate a property value associated with the
material based upon the signals produced by the radiation detector and
the moisture property detector.

[0034] Initially, it is noted that there are two dominant interacting
mechanisms with matter for gamma radiation with energies less than 1 mega
electronvolt (MeV). For gamma ray energies less than 0.1 MeV, the
dominant interaction is photoelectric absorption (PE) wherein the entire
gamma radiation energy is provided for ejecting an electron from the
atomic orbit. For common elements found in construction materials, the
dominant interaction for gamma radiation energies greater than 0.2 MeV,
is Compton scattering (CS), the scattering of photons by electrons in the
atoms.

[0035] To explain the photon interaction types, consider a nuclear density
gauge that includes a gamma radiation source for producing a parallel
beam of photons with discrete energies having a uniform distribution. The
beam of photons are directed through a sample material. If the photon
interaction mechanism is essentially photoelectric absorption (depending
on the reaction cross-section or probability, which is specific to the
sample material), some of the photons are lost from the radiation beam
due to absorption. Because of the absorption, the photon energy spectrum
will vary from location to location in the sample material. Since cross
sections are higher for low energy photons (i.e., energy less than 0.1
MeV), the low energy part of the spectrum shows a decreasing response or
dip. The spectrum dip increases as the effective atomic number of the
material increases. If the photon interaction is essentially Compton
scattering, the photon energy spectrum will vary from location to
location in the material with a variation of counts or flux in the high
energy portion of the spectrum. Counts decrease as the electron density
increases and vice versa and are mostly independent of the elemental
composition of the sample material.

[0036] Since there is a unique relationship between electron density and
material density for most materials, the gamma radiation flux may be used
for measuring material density. Gamma radiation flux decreases in an
exponential manner with the increase in material density. Nuclear density
gauges according to the subject matter described herein are operable to
expose sample material to gamma radiation, determine photon counts of
radiation emitted from the sample material and within a predetermined
energy level, and determine density of the sample material based upon the
photon counts with the predetermined energy level. In practice, both
photoelectric absorption and Compton scattering exist with some
probability in the entire energy range. Therefore, the energy spectral
features (i.e., features in the low energy and high energy portions) can
be used to accurately measure the material density.

[0037] For a material with an effective atomic number Z and atomic mass A,
the electron density ρe is provided by the following equation
(wherein ρ represents the mass density, and NA represents
Avagadro's number):

ρe=ρ(Z/A)NA

In general, (Z/A) for a majority of the elements in construction or road
paving materials is 0.5. One notable exception is H, where Z/A is about
1. When Z/A is assumed to be 0.5, the density can be determined based
upon Compton scattering.

[0038] Soils used for construction and asphalt typically have distinctly
different elemental composition. For gamma radiation-based density
measurements, construction soil material and asphalt material may be
treated as different classes of materials because of their different
elemental composition. For soil, because of the wide variance in water
content, separate measurement of the water content may be used for
improving the accuracy of density measurements. In nuclear density
gauges, an electromagnetic-based system or a neutron-based system may be
used for determining a moisture property of the sample material, such as
water content or other moisture content.

[0039] When using gamma radiation-based nuclear density gauges for density
measurements, the determination of material differences in samples can be
challenging. The material that effects gamma radiation propagation is the
elemental composition, or the amount of various chemical elements
composing the sample material. The density precision demanded by industry
can be as high as 0.65%. Therefore, a minor deviation of Z/A from 0.5 may
require correction to meet industry density precision requirements.

[0041] Table 2 below shows exemplary chemical elements for three limestone
mixes and three granite mixes used for hot mixed asphalt in the road
construction industry. It is notable that most of the limestone
aggregates have similar Z/A values, and most of the granite type
aggregates have a similar Z/A value. The differences of both values to
0.5 are significant enough to meet industry demand.

[0042] In one embodiment of the subject matter described herein, a Cs-137
gamma radiation source is used in a nuclear density gauge for measuring
the density of a sample material. However, other suitable gamma radiation
sources with different primary energy levels may be employed, such as a
Co-60, Ra-60, or any other suitable isotope gamma radiation source for
example. Gamma radiation interacting with a sample material may be
measured by an energy-selective, gamma radiation detector, which may be
operable to detect gamma radiation in one or more predetermined energy
spectrums. For example, an energy-selective scintillation detector may be
used, such as a sodium iodide (NaI) crystal mounted on a photomultiplier
tube (PMT) for detecting gamma radiation in a predetermined energy
spectrum.

[0043] As stated above, a nuclear density gauge according to the subject
matter described herein may include a moisture property detector for
determining a moisture property of a sample material, such as soil. The
presence of a significant fraction of water or various other moisture in
soil may require correction to manage an anomalous Z/A value of hydrogen.
The moisture content of the sample material may be measured using the
moisture property gauge and used for correcting density measurements
obtained by a nuclear density gauge.

[0044] An exemplary moisture property detector is a neutron-based
detector, which is sensitive to low energy neutrons from a material. An
example is the gas tube detector filled with a gas of He-3 and CO2,
known as an He-3 tube. The low energy neutrons have interacted with the
hydrogen contained in water in the material. The detector may count the
number of slow moving neutrons. The count of slow moving neutrons may
correspond with a moisture property of the material. Thus, a moisture
property of the material may be determined based on, the neutron count.
Neutron-based detectors are calibrated at a construction worksite,
because the chemical composition of the soils containing hydrogen, and
not associated with water, may affect the measurement results.

[0046] An electromagnetic device may measure the permittivity of a
material and use the dielectric constant and conductivity to estimate the
density of the material. Electromagnetic techniques are sensitive to the
chemical composition of the material, because permittivity is a result of
molecular bonding, soil chemistry, texture, temperature, water content,
void ratio, shape, and history of the material. Fundamentally, the
electromagnetic fields respond to the "dipoles per unit volume" or the
chemical composition per unit volume. Hence, within even a small area of
measurement, there may be significant changes in material properties such
as texture, water content, clay content, mineralogy, and gradation. As a
result, the electromagnetic device may require frequent calibration.

[0047] Nuclear techniques are also a function of chemical composition as a
result of photoelectric effects and extraneous hydrogen not associated
with water. However, the errors associated with nuclear techniques are
very forgiving as compared to dielectric spectroscopy techniques. For
neutron water measurements, hydrogen bonding from other chemical
compositions is also measured, such as soils that are heavy in mica,
salt, iron oxide, etc.

[0048] Signals produced by a radiation detector and an electrical property
detector may be used by a material property calculation function for
calculating a property value associated with a material. The signal
produced by the radiation detector may represent an energy level of
detected radiation from the material. The signal produced by the
electrical property detector may represent a moisture property of the
material. The calculated property value may be a density of the material.
The calculation of the material property values by the material property
calculation function may be implemented by a suitably programmed
processor or by any other functionally equivalent device, such as an
application specific integrated circuit (ASIC) or a general purpose
computer, having suitable hardware, software, and/or firmware components.

[0049] In one example, soil content and losses can be estimated by
inspecting dielectric constant dispersion over a microwave bandwidth from
DC to a few GHz. FIGS. 1A-1C are graphs illustrating examples of
dielectric dispersion for a variety of soils. In particular, FIG. 1A
shows a comparison of dielectric constants of clay material (cohesive
soil) and non-clay material (non-cohesive soil) over different
frequencies. FIG. 1B shows dielectric constant dispersion of several
different types of clays. FIG. 1C shows the dielectric dispersion of the
conductivity and dielectric constant of cohesive soil.

[0050] Information regarding dielectric constant dispersion for known
materials may be used in the subject matter described herein for
selecting calibration curves for radiation detectors and moisture
property detectors. Further, the subject matter described herein may be a
combination asphalt and soils gauge having operability to measure asphalt
layers in a backscatter mode and soils in a transmission mode. Further,
for example, a fringing field planar detector may be attached to a bottom
surface of the gauge for simultaneously measuring electromagnetic density
and nuclear density. In this mode, the nuclear component can calibrate
the electromagnetic detectors in the field for improving the speed of
access to a capacitance asphalt density indicator.

[0051] The combination of a radiation source/detector and a moisture
property detector according to the subject matter described herein may
operate in a transmission mode and/or a backscatter mode. The moisture
property may be measured using a surface technique, direct transmission,
downhole technique, or a technique using a fringing field capacitors,
time domain reflectometry (TDR), microwave reflection, microwave
transmission, real and imaginary impedance measurements, phase shift,
absorption, and spectroscopic analysis. The sensor may be physically
integrated into the surface instrument. Alternatively, the sensor may be
a stand-alone moisture sensor linked electronically to the surface gauge.
An example of a stand-alone system is a moisture sensor integrated into a
drill rod. In a use of this exemplary gauge, a drill rod and hammer may
be used to punch a hole in the soil to make a pathway for insertion of
the source rod.

[0052] Nuclear density measurements may be used to obtain bulk density,
which may be derived using the following equation (wherein p represents
bulk density, M represents mass, V represents volume, Mw represents
the mass of water, and MS represents the mass of soil):

wherein dry density MS/V is provided by the following equation
(wherein ρd represents the dry density):

ρd=ρ/(1+w)

[0053] Alternatively, a measurement of the volumetric water content may be
determined using the following equation (wherein θ represents the
volumetric water content, VW represents the volume of water, and
Vt represents the total volume):

θ=VWVt

The volumetric water content may be converted to pounds per cubic foot
(PCF), where it may be subtracted from the wet density moisture
measurement provided by the following equation (wherein γw
represents the density of water in proper units):

ρd=ρ-γwθ

[0054] Variables affecting the electrical response of soils include
texture, structure, soluble salts, water content, temperature, density,
and frequency. The following equation provides a general relationship for
volumetric water content (wherein .di-elect cons. represents
permittivity, A=-5.3×10-2, B=2.92×10-2,
C=-5.5×10-4, and D=4.3×10-6):

θ=A+B.di-elect cons.+C.di-elect cons.2+D.di-elect cons.3

In this equation, permittivity E is the real part and is a single value
measured over the frequency content of the time domain signal. A similar
equation may be found using the fringing field capacitor at a single
frequency or over an average of frequencies. For results, the moisture
detector may be calibrated to the soil type directly from the field.

[0055]FIG. 2 is a vertical cross-sectional view of a nuclear density
gauge 200 for measuring the density of material according to an
embodiment of the subject matter described herein. Gauge 200 may be
operable to accurately determine the density of a sample material, such
as soil, asphalt, concrete, or any other suitable construction and/or
paving material. For example, soil may be measured in a transmission
mode, and asphalt may be measured in a backscatter mode. Referring to
FIG. 2, gauge 200 may include a primary gamma radiation source 202 and a
gamma radiation detector 204. Radiation source 202 may be any suitable
radiation source, such as a 300 micro Curie Cs-137 gamma radiation
source. Gamma radiation detector 204 may be any suitable type of
detector, such as a gamma-ray scintillation detector of the type having a
sodium iodide (NaI) crystal 206 mounted on a photomultiplier tube 208. A
gamma radiation detector of scintillation-type is an energy selective
detector. Radiation detector 204 may be located adjacent to a base plate
210. When gamma radiation strikes NaI crystal 206, photons are released,
varying in intensity corresponding to the energy level of the gamma
radiation. Photomultiplier tube 208 detects the photons and converts them
to electrical signals which, in turn, are communicated to an amplifier
for amplifying the electrical signals. Further, the amplified signals may
be directed, via an electrical conductor, to a printed circuit board
(PCB) 211, where the signals may be processed.

[0056] PCB 211 may include suitable hardware (e.g., a multi-channel
analyzer (MCA)), software, and/or firmware components for processing the
amplified signals211. PCB may include an analog-to-digital converter for
transforming the amplified analog signals into digital signals
quantifying the energy level of the gamma radiation (photon) energy. The
output of the analog-to-digital converter is directed to an analyzer
device operable to accumulate the number of gamma radiation (photon)
counts of different energy levels into a plurality of channels, each
channel corresponding to a portion of the energy level spectrum. For
purposes of density calculation, only a predetermined portion of the
overall energy spectrum detected by the detectors is considered. Thus,
only the accumulated counts from one or more of the channels
corresponding to this predetermined portion are considered for the
density calculation. The channel output may be used for density
calculations, as described in further detail herein.

[0057] Gauge 200 may be adapted to position radiation source 202 in an
interior of a sample material 212 to be tested. For example, radiation
source 202 may be contained within a distal end of a movable, cylindrical
source rod 214, which is adapted to be moved in the vertical directions
indicated by arrows 216 and 218. Source rod 214 extends into a vertical
cavity 220 in a gauge housing 222. Source rod 214 may be restricted to
movement in the vertical directions by guides 224, a support tower 226,
and an index rod 228. Guides 224 may include bearings that are
operatively positioned to guide source rod 214 through cavity 220 in
gauge housing 222. Source rod 214 may be vertically extended and
retracted to a plurality of predetermined source rod positions so as to
change the spatial relationship between radiation source 202 and detector
204. The plurality of predetermined source rod positions may include a
backscatter position and a plurality of transmission positions, wherein
radiation source 202 is positioned below base plate 210 of gauge housing
222.

[0058] Index rod 228 may be operatively positioned adjacent to source rod
214 for extending and retracting source rod 214. Index rod 228 may
include a plurality of notches 230. Each notch 230 corresponds to a
predetermined source rod position. For example, one notch may correspond
to a "safe" position wherein radiation source 202 is raised and shielded
from the sample material. Gauge 200 is shown in the safe position in FIG.
2. The safe position may be used to determine the standard count in a
background measurement mode, as described herein. Another notch may
correspond to the backscatter mode wherein radiation source 202 is
located adjacent to the surface of the sample material underlying gauge
200. Index rod 228 may include a flat side where a resistive depth strip
(not shown) may be affixed. Other exemplary depth indicators include Hall
effect devices, laser position indicators, and mechanical position
indicators.

[0059] Source rod 214 may be affixed to a handle 232 for manual vertical
movement of source rod 214 by an operator. Index rod 228 extends into a
cavity 234 in handle 232. Handle 232 further includes an indexer 236
operatively positioned for engaging notches 230 of index rod 228 in order
to temporarily affix source rod 214 in one of the predetermined
positions. Indexer 236 is biased into engagement with notches 230. In
particular, indexer 236 may be biased into engagement by a spring 238. A
trigger 240 allows the operator to move indexer 236 into and out of
engagement with notches 230.

[0060] Source rod 214 may be positioned in a safe position as shown in
FIG. 2 and secured for positioning source 202 within a safety shield 242.
When in the safe position, safety shield 242 contains the gamma rays
emitted by source 202 minimizes the operator's exposure to radiation.
Safety shield 242 may be made of tungsten, lead, or any other suitable
radiation shielding material.

[0061] Gauge 200 may also include additional shielding for preventing
undesirable emission of gamma radiation from gamma radiation source 202.
A stationary shield 244, safety shield 242, and a sliding block shield
246 may be included within gauge 200 and positioned for stopping emitted
photons from directly reaching detectors of gauge 200. Shields 242 and
246 may be made of tungsten. Alternatively, shields 242, 244, and 246 may
be made of any other suitable shielding material. Safety shield 242 may
include a hole formed therein and through which rod 214 and source 202
may pass. In the safe position, source 202 may be positioned in the
interior of safety shield 242 for preventing photons of source 202 from
reaching the detectors of gauge 200. Stationary shield 244 may be
positioned for preventing photons from reaching the detectors through
pathways through the interior of gauge 200.

[0062] Detector 204 may be energy-calibrated by use of another gamma
radiation source 248. Radiation source 248 may be positioned within an
aluminum support 249 and positioned adjacent to base plate 210 and
detector 204. In one example, radiation source 248 may be a 1 to 2 micro
Curie Cs-137 gamma radiation source. Radiation source 248 may be used to
energy calibrate detector 204 for managing environmental effects, such as
temperature. In one example, radiation source 248 may produce main energy
peaks of about 33 and 662 kilo electronvolts (keV). The energy peaks
produced by radiation source 248 may be used for calibrating detector 204
for use as a multi-channel spectrum analyzer, as described in further
detail herein. In an alternative embodiment, a small leak hole may be
provided in cylindrical shield 242 to allow the energy from gamma
radiation source 202 to radiate towards detector 204 for
energy-calibrating detector 204.

[0063] Further, gauge 200 may include a moisture property detector 250
operable to determine a moisture property of sample material 212. In
particular, detector 250 may measure the permittivity of sample material
212 and use the dielectric constant and conductivity to estimate the
moisture property of sample material 212. The following exemplary
moisture properties, alone or combinations thereof, may be detected by a
moisture property detector for use in determining the density of a sample
material: permittivity, resistivity, dielectric constant, conductivity,
permeability, dispersive properties, change in dielectric constant with
frequency, change in conductivity with frequency, the real part of
permittivity (i.e., dielectric constant), the imaginary part of
permittivity, and combinations thereof.

[0064] In this example, moisture property detector 250 may include a
moisture signal source 252, a moisture signal detector 254, and a PCB
256. Moisture property detector 250, as an electromagnetic detector, may
operate in a far field radiation mode, a near field mode, a passive
fringing mode, or by coupling the fields from source to receiver through
sample material 212. Signal source 252 may generate an electromagnetic
field and be positioned near a surface of sample material 212 such that
the electromagnetic field extends into sample material 212.
Alternatively, signal source 252 and/or detector 254 may be positioned
within an interior of sample material 212 via source rod 214. In one
embodiment, a combination source/detector device may be attached to a
source rod for obtaining depth information. In another embodiment, a
combination source/detector device may be external to the gauge and
detached.

[0065] Moisture signal detector 254 may detect at least a portion of the
electromagnetic field from sample material 212 that was produced by
source 252. A frequency and/or time domain technique may be used for
determining a moisture property. The electromagnetic field may range from
direct current (DC) to microwave. Exemplary techniques for use in
determining a moisture property include using fringing field capacitors
to produce an electromagnetic field; time domain reflectometry
techniques; single-frequency moisture techniques; sweeping-frequency
moisture techniques; microwave absorption techniques; and microwave phase
shift techniques. Further, suitable moisture signal detectors include
detectors operable to measure the real and imaginary parts of a
dielectric constant at a single frequency, multiple frequencies,
continuous sweeps of frequencies, and/or chirps of frequency content. In
the time domain, direct steps or pulses may be produced by a signal
source and detected by a detector for determining a moisture property. In
one example, source rod 214 may be pulsed, the response received at
detector 254, and the phase velocity calculated from the time-distance
information. Further, a fast Fourier transform (FFT) technique may be
applied to the frequency and time domains for determining a moisture
property. The conductivity and permittivity of sample material b may be
determined based on the detected electromagnetic field.

[0067] PCB 256 may be in operable communication with signal source 252 and
detector 254. PCB 256 may include suitable hardware, software, and/or
firmware components for control of signal source 252 and detector 254. In
particular, PCB 256 may control signal source 252 to generate an
electromagnetic field. For example, PCB 256 may supply power to circuitry
of signal source 252 for generating a predetermined electromagnetic
field. Further, PCB 256 may be operable to receive a signal from detector
254 representing detected electromagnetic fields via a coaxial cable 262.
Based on the signal representation, PCB 256 may determine a moisture
property of sample material 212. For example, measurement of the
magnitude and phase of reflected signals may provide an impedance that is
a function of constitutive parameters permittivity and permeability of
the material. An impedance bridge may be used for obtaining the complex
impedance at lower frequencies. For higher frequencies, reflectometers
incorporating mixers or detectors (e.g., magnitude and phase integrated
circuits, manufactured by Analog Devices, Inc. of Norwood, Masschusetts)
may be used. For time domain reflectometry (TDR), diode techniques and
timing/recording circuitry may be used to obtain voltage as a function of
time.

[0069] Moisture measurement may rely on single variable or multi-variable
equations. For example, water may be detected using one variable such as
the relative dielectric constant .di-elect cons.r. Interfacial
polarization is an important property response for heterogeneous
materials. Further, the relaxation frequency of some soils is on the
order of 27 MHz. At lower frequencies, the measured dielectric constant
has the effects of the Maxwell Wagner phenomenon leading to errors in the
water measurement that are also a function of temperature. Other
exemplary variables include conductivity, permittivity, and the disperson
of the change in conductivity and the change in permittivity with
frequency. Further, for example, the relaxation frequency of some soils
is on the order of 27 MHz.

[0070] In one example, the capacitance of a fringing field detector is
measured using a feedback loop in an oscillator circuit. The frequency is
provided by the following equation (wherein Ceff represents the
effective capacitance including the surrounding medium, parasitics in the
circuitry, and nominal capacitances in the tank circuit, and L represents
the inductance):

2πF=1/(sqrt(LCeff))

The ratio between a reference frequency and the frequency with the
fringing field capacitor switched may be calibrated against moisture. The
sensitivity of the measurement at these frequencies due to salt
concentrations should be considered. The end result is that chemical
composition errors must be corrected, leading to many different
calibration curves for the soil types. Further, discussion is provided in
U.S. Pat. Nos. 4,924,173; and 5,260,666, each of which are incorporated
herein by reference in their entireties.

[0071] Microwave-based moisture property detectors may be advantageous,
for example, because such detectors can perform density-independent
moisture measurements. Such detectors may be advantageous over
neutron-based moisture property detectors, because neutron-based
detectors are density dependent. Further, it is desirable to reduce the
use of neutron sources because of NRC regulations and fees associated
with neutron sources.

[0072] Density-independent moisture measurements may be made based on a
two-parameter measurement of attenuation (or magnitude) and phase shift
in a transmission- or reflection-type mode. Alternatively,
density-independent moisture measurements may be made using microwaves at
a single frequency. A two-parameter method may be implemented by
comparing the real and imaginary parts of the dielectric constant, as
shown in the following equation (wherein E represents the dielectric
constant):

.di-elect cons.=.di-elect cons.(ω)'-j.di-elect cons.(ω)''

[0073] A density independent calibration factor A(ψ) (wherein ψ is
the wet-based volumetric water content) may be used for canceling density
components. The principle of density-independent moisture measurements is
based on both the real and imaginary part of the dielectric constant
being related to dry material and water constituents, which change as a
function of density. Density components may be empirically canceled by
combining .di-elect cons.(ρd, ψ)' and j.di-elect
cons.(ρd, ψ)'' in the following equation:

The above equation assumes that .di-elect cons.(ω)' and .di-elect
cons.(ω)'' are linearly independent functions of ρd and
ψ.

[0074] The loss tangent .di-elect cons.'/.di-elect cons.'' may describe
the material interaction and response. The behavior of the complex
permittivity implies that normalizing both .di-elect cons.(ω)' and
j.di-elect cons.(ω)'' with density may reduce density effects.
Further, data pairs may be normalized with bulk density as functions of
temperature and moisture content. The following equation provides a
measure of bulk density without prior knowledge of moisture content or
temperature given that moisture density relationships are independent
(wherein af represents slope, k represents intercept, af is
related to the frequency, and k related to the dry dielectric):

.di-elect cons.''/ρ=af(.di-elect cons.'/ρ-k)

Alternatively, the following equation provides a measure of bulk density:

P=(af.di-elect cons.'-.di-elect cons.'')/kaf

[0075] At high frequencies, water is the dominant factor associated with
energy loss related to .di-elect cons.'' in the material, and the energy
storage is related to .di-elect cons.'. Thus, a density-independent
function for water content is based on the loss tangent .di-elect
cons.''/.di-elect cons.'. Therefore, again, by normalizing the loss
tangent by the density provided by the above equation results in the
following equation:

Here, the constant kaf is omitted, and the loss tangent has been
normalized, resulting in a moisture function with reduced density
effects. Experimentally, for granular materials, it has been found that
ξ is linear with moisture content kaf is a function of the
measurement frequency and remains constant for data pairs of .di-elect
cons.' and .di-elect cons.' when they have been normalized by density.

[0076] Based on experimental results, it can be shown that, as temperature
increases, the bound water becomes easier to rotate and the dielectric
constant increases. Thus, for the water measurement, temperature
correction may be necessary.

[0077] Since ξ is a function of moisture content with the density
effects removed, and since it is experimentally found to be linearly
related to moisture, calibration as a function of moisture and
temperature can be implemented by fitting to the following linear
equation:

ξ=A*M+B(T')

In this equation, the intercept B increases with temperature, but the
slope A is constant. For granular materials, the following equation was
empirically derived (wherein temperature is measured in Celsius):

B(T)=9.77×10-4*T+0.206

The moisture content may then be determined using the following equation:

%M=( ξ(af.di-elect cons.',.di-elect cons.'')-B(T))/A

In one embodiment, samples of soil may be extracted from the field and
fit to this equation as a function of moisture yielding the constants A
and B at a particular temperature. Generic curves may also be defined
whereby a field offset is performed in use. Therefore, any moisture
property detector operable to measure the real and/or imaginary portions
of the dielectric constant of a material at a single frequency, multiple
frequencies, or continuous sweeps of frequencies, chirps of frequency
content, on the surface or down-hole can be incorporated into embodiments
of the subject matter described herein.

[0078] Microwaves are more sensitive to free water than bound water but
are also a function of the constituents of the chemical makeup of the dry
mass and water mass mixture. However, a dry mass and water mass mixture
is less susceptible to ionic motion and DC conductivity when considering
the following equation:

The higher frequencies reduce the effects of DC conductivity and measure
more of the dielectric permittivity. However, soil specific calibrations
may be necessary. The differences in the calibrations are much smaller
than their low frequency counterparts. Thus, if the material changes
slightly without a gauge operator's knowledge, suitable results may still
be obtained. Therefore, the microwave electromagnetic techniques have
soil specific calibrations or offsets that may be required when comparing
sandy foams to clay classes of soils.

[0079] Sliding block shield 246 is configured to be slidable within a
chamber 264 and associated with a spring 266, which is adapted for
biasing shield 246 in a direction towards an interior of safety shield
242. In the safe position, at least a portion of shield 246 is positioned
in the interior of safety shield 242 for preventing photons emitted by
source 202 from passing through safety shield 242. On movement or rod 214
in the direction indicated by arrow 218 towards the position for
transmission mode, block shield 246 is pushed by an end of rod 214 away
from the interior of safety shield b and against the biasing direction of
spring 266. Shield 246 may include a beveled portion 268 adapted to
engage an end of rod 214 for pushing shield 246 away from the interior of
safety shield 242 such that rod 214 and source 202 may move into the
position for the transmission mode. Movement of shield 246 away from the
interior of safety shield 242 compresses spring 266.

[0080] FIG. 3 is a vertical cross-sectional view of nuclear density gauge
200 configured in a transmission mode for measuring the density of sample
material 212 according to an embodiment of the subject matter described
herein. Referring to FIG. 3, in the transmission mode, radiation source
202 may be positioned in an interior of sample material 212 for emitting
radiation from the interior of sample material 212. In the transmission
mode, radiation source 202 may emit radiation through sample material 212
for detection by radiation detector 204. Further, PCB 211 may produce a
signal representing an energy level of the detected radiation. Moisture
property detector 250 may determine a moisture property of sample
material 212 and produce a signal representing the moisture property. A
PCB 269 may include a material property calculation function (MPC) 270
configured to calculate a property value associated with sample material
212 based upon the signals produced by radiation detector 204 and
moisture property detector 250.

[0081] MPC 270 may include suitable hardware, software, and/or firmware
components for implementing density measurement and calibration
procedures according to the subject matter described herein. MPC 270 may
include one or more processors and memory components. Exemplary MPC
components include one or more of pre-amplifiers, spectroscopic grade
Gaussian amplifiers, peak detectors, and analog-to-digital converters
(ADCs) for performing the processes described herein. Procedure status,
feedback, and density measurement information may be presented to an
operator via one or more interfaces of gauge 200.

[0082] A nuclear density gauge may be calibrated for density and moisture
measurements. In one embodiment, measurements of the dielectric constants
of different synthetic materials are fit to a calibration curve. The
materials may be selected to represent materials found in the
construction field. Solid metal blocks of known properties may be used
for calibrating a nuclear density gauge. Exemplary metal blocks for use
in calibration include a magnesium (Mg) block (MG), a Mg and aluminum
(A1)-laminated block (MA), an Al block (AL), and a Mg and
polyethylene-laminated block (MP). The MG, MA, and AL set may be used for
density calibration. The MG and MP set may be used for moisture
calibration. It is noted that the gravimetric density of Mg is about 110
pounds per cubic foot (PCF), Al is about 165 PCF, and MG and Al are about
135 PCF.

[0083] For density measurements, when calibrating a nuclear density gauge
for soil measurements, typical soils are assumed to have a Z/A of 0.5. To
emulate Z/A=0.5, the gravimetric density values of the calibration blocks
ρgrav may be normalized with respect to the Z/A value and used
with gamma radiation counts to determine calibration coefficients. A
calibration model is provided by the following equation (wherein CR is
the count ratio for the test sample, ρnorm is the normalized
density of the test sample, and A, B, and C are calibration
coefficients):

[0085] A direct gauge reading on a test material is relative to the Z/A
value used in the calibrations. For materials having significantly
different Z/A values, the gauge may be calibrated specifically for the
material.

[0086] For moisture example of laboratory calibration, a soil sample may
be removed from a field site. The soil sample is dried in an oven
according to ASTM standard 2216. Different amounts of water are added to
the dried soil, and the material is stored for a predetermined time
period. The soils are then compressed into a coaxial cylinder. Next, a
function of the water content and measurements of the permittivity are
obtained as a function of frequency over a broad band. The permittivity
is recorded as a function of frequency and temperature. The coaxial
cylinders are then weighed and dried to obtain the actual water and
density content. For single frequency measurements, the permittivity may
be normalized with density and corrected for temperature. The slope
af may be found using the equations described above. Further, by
using the equations described herein, the moisture equation may be
derived and programmed into the nuclear gauge for field use.

[0087] In field use, the calibration for specific materials is performed
by finding an offset to the gauge by comparing gauge readings to density
values as determined by a conventional method. For example, a sand cone
technique (ASTM standard D-1556) may be used for soils. In another
example, an operator may use the gauge to perform a measurement in the
field, and use an oven test according to the ASTM standard 2216 to
evaporate the water and obtain the moisture content in volumetric or
gravimetric units. The resulting value in this example may be used to
offset factory or laboratory calibration. In an example for asphalt, a
coring and water displacement technique (ASTM standard D-2726) may be
used.

[0088] In field calibrations, the nuclear density gauge may be positioned
on the soil. Typically, the soil is wet with different moisture contents.
Measurements of the real part of the dielectric constant may be obtained
as a function of the water content. The response is fit to a linear
equation, such as y=mx+b, wherein x is the response of the gauge. The
nuclear density gauge may be calibrated in steps similar to the steps
used for laboratory calibration, except for one or more of the following,
only the imaginary portion of the dielectric constant is used, only the
capacitance of a detector is used, only the resistance measurement is
used, only TDR is used, only frequency response is used, only the
relative dielectric constant is used, and only dispersion data is used.

[0089] As stated above, the presence of a significant fraction of water or
various other moisture in construction-type soil may require correction
to manage an anomalous Z/A value of hydrogen. The wet density of soil is
provided by the following equation (wherein WD represents the wet density
of soil, GD represents the gauge density (mass per unit volume) from
direct calibration, and M represents gauge moisture content (mass of
water per unit volume of moist soil)):

WD=GD-( 1/20)M

These corrections to direct nuclear gauge readings improve the accuracy
of the density estimate provided Compton scattering is the only
interaction mechanism for gamma radiation. Detected gamma radiation of
energies greater than 0.15 MeV meets this requirement for typical
construction materials.

[0090] Gas ionization detectors, such as Geiger Mueller detectors, may be
used in nuclear density gauges for gamma radiation or photon counting.
Such detectors have relatively higher detection efficiencies in the 0 to
0.2 MeV range than in the 0.2 MeV or higher range but cannot accurately
detect the color or energy of counted photons. The photon counts recorded
by such detectors also contain the attenuation effect of low energy gamma
radiation from photoelectric absorption. The model described above for
handling the Z/A effect may not be met. As a result, density accuracy may
be compromised.

[0091] A scintillation detector is an energy-selective detector operable
to selectively use gamma radiation energies above 0.15 MeV during gauge
calibration and measurement. The signal amplitude of a sodium iodide
crystal/PMT detector depends linearly on the detected photon energy. A
histogram of the number of detected photons versus energy signal
amplitude provides a gamma radiation spectrum. For a given photon energy,
the energy signal amplitude depends on the PMT signal gain and the
environmental temperature. Therefore, with no feedback control of the
detector, the position of key features of the spectrum (i.e., spectrum
peaks) vary with time. When counts in a particular energy window (or
range) are required, spectrum stabilization techniques may be used to
minimize the effects form short-term signal amplitude variability, as
described in further detail herein.

[0092] FIG. 4 is a flow chart illustrating an exemplary process by which
gauge 200 shown in FIGS. 2 and 3 may be initialized at the beginning of a
workday according to an embodiment of the subject matter described
herein. In this example, radiation detector 204 is calibrated for use as
a multi-channel spectrum analyzer. Referring to FIG. 4, the process
starts at block 400. In block 402, a high voltage power supply that is
connected to radiation detector 204 is turned on. For example, gauge 200
may include a battery 276 configured to supply power to radiation
detector 204. In block 404, a predetermined number of channels in the
energy spectrum of the radiation provided by radiation source 202 to
detector 204 may be set. In this example, the number of channels in the
spectrum is set to 512. In block 406, radiation source 202 is positioned
for emitting radiation. Radiation detector 204 may detect radiation
emitted by radiation source 202. As stated above, radiation source 202
may be Cs-137 gamma radiation source for producing energy peaks of about
33 and 662 keV. The energy peaks produced by radiation source 202 may be
used for calibrating detector 204. During calibration, source rod 214 may
be positioned in a safety mode such that radiation detector 204 is
shielded from radiation source 202.

[0093] In block 408, an amplifier gain of radiation detector 204 is set to
a default value. Further, in block 410, a data collection time of
radiation detector 204 is set to a predetermined time period (e.g., 20
seconds). In block 412, the process waits a predetermined time period
(e.g., between about two and five minutes). After detector 204 has warmed
up, a radiation count is obtained from the underlying material.

[0094] Next, in blocks 414-420, an amplifier gain of radiation detector
204 may be adjusted until a centroid channel is between 208 and 212. The
amplifier gain may be set such that the centroid of the 662 keV gamma
radiation peak from Cs-137 is in the middle of the 208 to 222 channel
window. As the gauge is used, depending on the environment, the centroid
may move in the acceptance window defined by channels 200 and 220. Prior
use for measurements, MPC 207 may verify that the centroid lies in this
channel window. If MPC 207 determines that the centroid lies outside this
channel window, the centroid may be moved back to the mid area of the
channel window defined by channels 208 and 212 in about 20 seconds, and a
message may be displayed on a display screen 274 of gauge 200 indicating
the delay. In a typical use, the gain may need to be moved to center the
peak approximately one or two times per day. During idle times, MPC 207
may implement an active routine for changing gain.

[0095] In particular, in block 414, data is collected from radiation
detector 204. For example, radiation detector 204 may communicate
acquired data and communicate the data to MPC 270. MPC 270 may calculate
the centroid channel for the 662 keV energy peak (block 416). In block
418, it is determined whether the centroid channel is between 208 and
212. If it is determined that the centroid channel is not between 208 and
212, the amplifier gain is changed (block 420). Otherwise, if it is
determined that the centroid channel is between 208 and 212 the process
stops at block 422. Now, radiation detector 204 is ready for
measurements.

[0096] The centroid may move in the acceptance window during normal
temperature conditions in the field. Further, when the gauge is used on
hot asphalt, the increase in temperature of the radiation detector can
result in a centroid location being outside of the acceptance window. If
the centroid location is found to be outside of the acceptance window,
the system gain may be adjusted to center the centroid at channel 210.
The system gain may be changed by adjusting either the gain of the
shaping amplifier or the voltage supplied to the photomultiplier tube of
the radiation detector.

[0097] A detected energy level is analyzed when the location of a
predetermined energy level peak is within an acceptance window. For
example, an energy level peak of 662 keV must be within an acceptance
window of between channels 200 and 220 within a 512 channel spectrum.
FIG. 5 is a flow chart of an exemplary process for determining detector
counts within an energy window defined by energy values Ei and Ef
according to an embodiment of the subject matter described herein. The
process of FIG. 5 may be implemented after the gauge has been
initialized, for example, by the exemplary process of FIG. 4. Referring
to FIG. 5, in block 500, a data collection time of radiation detector is
set to a predetermined time period (e.g., 15 or 30 seconds). Next, in
block 502, energy values Ei and Ef are obtained. In block 504, data is
collected from radiation detector 204.

[0098] Next, in block 506, MPC 270 may calculate a centroid channel C2 for
the 662 keV energy peak. MPC 270 may determine whether the centroid
channel C2 is between channels 200 and 220 (block 508). If the centroid
channel C2 is not between channels 200 and 220, the process can adjust
the amplifier gain of radiation detector 204 according to a process
similar to that described with respect to blocks 414-420 of FIG. 4 (block
510). Otherwise, if the centroid channel C2 is not between channels 200
and 220, the process proceeds to block 512.

[0099] In block 512, using a look-up table, a centroid channel C1 may be
found for the energy level peak of 33 keV. Next, in block 514, MPC 270
may solve for coefficients A0 and A1 for calibration equation E=A0+A1*C,
a first order energy calibration where C is the channel number. In block
516, MPC 270 may solve for channel numbers Ci and Cf corresponding to
energy values Ei and Ef, respectively. MPC 270 may then find counts CW
corresponding to energy values Ei and Ef (block 518). CW is the total
counts of channels Ci to Cf, where a count value is associated with each
channel. Counts CW may be used for density calculation processes, as
described in detail herein. Since channel numbers are integer values,
fractional channel numbers may be handled in a manner as an
analog-to-digital converter digitizes signals.

[0100] Typically, sample material contains natural radioactivity, such as
natural radio isotopes of K, U, and Th. When using a low activity gamma
radiation source, the natural radioactivity manifests itself as noise.
Since the signal-to-noise ratio is low and the magnitude of the noise
varies from material to material, a separate measurement of the noise
(background) is required for maintaining the accuracy of the measurement.
Nuclear gauge 200 is shown in FIG. 2 configured in a background
measurement mode for managing noise. As stated above, in this
configuration, shields 242, 244, and 246 prevent gamma radiation produced
by radiation source 202 from reaching radiation detector 204. The gamma
radiation reaching radiation detector 204 is produced by material sample
212 (natural radioactivity or background) and stabilization source 248.
Since the small stabilization source 248 is positioned near radiation
detector 204, the background spectrum can be measured with adequate
accuracy. Background counts are not necessary for 8 milli Curie
Geiger-Mueller detector-based instruments, because the signal-to-noise
ratio is high.

[0101] Nuclear density gauge 200 is operable in a backscatter mode for
measuring asphalt layers. FIG. 6 is a vertical cross-sectional view of
nuclear density gauge 200 for measuring the density of asphalt layers
according to an embodiment of the subject matter described herein. In the
backscatter mode, source rod 214 is positioned such that radiation source
202 is on a surface of an asphalt layer 600.

[0102] Components of a nuclear density gauge operable in a backscatter
mode were used for demonstrating the functionality of its use as a
transmission gauge. The gauge components were positioned on a
magnesium/aluminum (Mg/Al) standard calibration block of size
24''×17''×14''. The gauge components included a 300 micro
Curie Cs-137 gamma radiation source fixed on a source plate. The base of
the gauge included a gamma radiation detector having a NaI crystal
mounted on a photomultiplier tube. PC-based electronics were used for
data acquisition.

[0103] Further, a source plate was attached to a 0.25-inch thick
14''×14'' aluminum mounting bracket having an open slot with screw
hole positions. The aluminum plate was attached to the 17''×14''
side of each metal calibration block. The source plate was also attached
to the aluminum plate so that the source is 2'', 4'', 6'', 8'', 10'', and
12'' below the top surface (a 24''×17'' surface) of the calibration
block. Each radiation source position is called an operating mode.

[0104] Standard metal calibration blocks made of Mg, Mg/Al, and Al were
used for calibrating the gauge. A standard count was used to compensate
for the decrease of the gamma radiation count over time due to
radioactive decay and other variations. In this experiment, counts for
the gauge operating in the backscatter mode and placed on the Mg block
was used as the standard count.

[0105] For gauge calibration, data was collected for each of the operating
modes, wherein the radiation source is positioned at 2'', 4'', 6'', 8'',
10'', and 12'' below the top surface of the calibration block. A four
minute count time was selected for the calibration of the six operating
modes. The net counts in the energy range from 150 to 800 keV were used.
Further, radiation spectra were taken on the Mg block, the Mg/Al block,
and the Al block without the radiation source for obtaining gamma
radiation background.

[0106] In backscatter mode experiments, the radiation source was
positioned about 2'' from the radiation detector and about 7'' from the
radiation detector. It is noted that, in actual use, the radiation source
and the radiation detector are in a fixed position with respect to one
another. For each operating mode position, the transmission mode was
tested with the radiation source near the detector in the Mg block, the
Mg/Al block, and the Al block. For obtaining the standard count, the
gauge was configured in the backscatter mode with and without the gamma
radiation source being positioned on the Mg block. FIGS. 7 and 8 are
graphs of experimentation results showing gamma radiation spectra for the
standard count and the 4-inch operating mode, respectively.

[0107] In one embodiment, the mathematical model used for calibrating a
nuclear density gauge is provided by the following equation (wherein, CR
represents the count ratio, and A, B, and C represent calibration
constants):

CR=A*exp(-B*Density)-C

CR is defined as the ratio of the net counts for a mode on a block of
density ρ to the net standard count. For example, for a 6''
transmission mode on a Mg/Al block, the net count is the difference of
the counts for the gauge with the gamma radiation source on the block and
the gauge without the gamma radiation source on the block. The net
standard count is the difference of the counts for the gauge in the
backscatter mode on the Mg/Al block with the gamma radiation source and
without the gamma radiation source. Table 5 below shows the calibration
constants for the six operating modes.

[0108] In one example of gauge 200 being used in the transmission mode,
counts in the energy interval from 150 to 800 keV for all spectra are
used for density calculation. In this example, count are normalized per
1-minute. For a 4-inch operating mode on an Mg block, the net count for
Mg is 341084. The net standard count is 2181382. Further, solving from
the equation CR=A*Exp(-B*Density)-C, density is provided by the following
equation:

Density=(-1/B)*Ln((Cr+C)/A)

The calibration constants A, B, and C for the 4-inch mode may be used
from Table 5 above, which may be stored in a memory associated with MPG
270. CR is provided by net count/standard count, which is 341084/2181482
in this example. By using the above equation, MPC 270 may determine that
the density is 109.4 PCF.

[0109] A nuclear density gauge according to the subject matter described
herein may operate in a backscatter mode for quality control and quality
assurance testing of asphalt pavements. Since asphalt pavements are
typically built with multiple layers including different mixes and
thicknesses, an accurate estimate of the density requires consideration
of the chemical composition, surface roughness, and the thickness of the
test layer.

[0110] Thickness of the top layer of an asphalt pavement may be specific
for the road construction project. For thin asphalt layers, a density
reading of the top layer may depend on the material type and density of
other asphalt layers below the top layer. The gauge reading may be
corrected if the bottom layer density is known accurately by using
features observed for layer-on-layer measurements. This correction method
is referred to a nomograph method and described in the Troxler Electronic
Laboratories, Inc. manual for the Model 3440 surface moisture density
gauge, produced by Troxler Electronic Laboratories, Inc., of Research
Triangle Park, N.C., the content of which is incorporated herein by
reference in its entirety. The Troxler Electronic Laboratories, Inc.
Model 4640 density gauge is another exemplary gauge for thin-layer
measurements, which uses two detector systems and the features observed
for layer-on-layer measurements.

[0111] When a photon is Compton scattered by an electron, the energy of
the photon depends upon the scattering angle. When a gamma radiation
source and detector are placed on a planar semi-infinite medium, the
single scattered photons for a given thickness have predetermined
energies. Such energy windows may be determined experimentally using
measurements of known thickness layers of materials, such as layers of
glass on an Mg/Al calibration block. The following energy bands may be
used to measure layers with thicknesses between 0.75'' and 2.5'':

[0112] 240 to 400 keV: 0.75'' to 1.25''

[0113] 220 to 400 keV: 1.25'' to 1.75''

[0114] 200 to 400 keV: 1.75'' to 2.0''

[0115] 180 to 400 keV: 2.0'' to 2.5''.

[0116] A dual layer structure made with dissimilar materials was formed in
the laboratory by placing glass slabs on an Mg/Al standard size block.
Next, gamma-ray spectra were acquired by placing a nuclear density gauge
on glass. FIG. 10 illustrates a graph of density measurements for various
gamma-ray energy bands as the glass thickness varied. The upper energy of
all bands was 400 keV. By using the energy band 80 to 400 keV, the gauge
measured a depth of about 3 inches. By using another energy band from
about 240 to 400 keV, the gauge measured a depth of about 1 inch.

[0117] When reading the density of thick layers, the window counts for
density determination contain gamma radiation of low energies. Such gamma
radiation is also absorbed by the photoelectric process to thereby cause
an error in density. The two major classes of aggregate types, granite
and limestone, have two different normalization constants for gamma
radiation in the Compton scattering region and varying degrees of
photoelectric absorption. As a result, the granite and limestone
aggregate types have distinct calibration curves. FIG. 11 illustrates a
graph of the calibration curves for granite and limestone mixes as
determined from experimentation. A prior identification of aggregate type
can improve the estimation of the density.

[0118] MPC 270 may use the gamma radiation spectrum for identifying
aggregate types. The photoelectric absorption process results in reduced
low-energy gamma radiation flux for materials with high atomic numbers
than that for materials with low atomic numbers. The average atomic
number of limestone mixes is higher than that for granite mixes.
Therefore, low energy counts in the spectrum normalized to density can be
used for aggregate type identification. For example, CL can represent the
counts in a low-energy window with low and high energy limits (EIl
and EIh), and CH can represent the counts in a high-energy window
with low and high energy limits (EIl and EHh). The ratio of
Rc=CL/CH may be used for aggregate identification. Based on experiments,
it was found that Rc<R0 for limestone mixes and that Rc>R0 for
granite mixes.

[0119] In the asphalt industry, the asphalt volume for density
determination may be defined in various ways. The material volume of the
asphalt may be determined by excluding surface texture. Further, a water
displacement technique and its variations may be used for density
measurements. Using gamma radiation techniques for density measurements
defines the asphalt volume including surface roughness. Therefore, direct
gamma radiation density values are lower than that measured by water
displacement techniques. Further, the air void content of asphaltic
materials (V) has a strong correlation to the surface roughness. If the
density difference between the water displacement and gamma radiation
techniques is dp, an empirical relationship between dp and V may be found
using the following equations:

dρ=B0g+B1g*V+B2g*V2 for granite, and

dρ=B0l+B1l*V+B22l*V2 for limestone.

[0120] Asphalt density measurements may be determined using gauge 200
configured in the backscatter mode shown in FIG. 6. FIG. 12 is a flow
chart illustrating an exemplary process for density measurements in a
backscatter mode using gauge 200 according to an embodiment of the
subject matter described herein. Referring to FIG. 12, in block 1200,
gauge 200 is positioned on a top surface of asphalt layer 600 as shown in
FIG. 6. Further, source rod 214 is positioned in the backscatter mode
such that radiation source 202 is positioned near the top surface of
asphalt layer 600. Further, in the backscatter mode, sliding block shield
246 is moved in the backscatter mode such that radiation source 202 can
emit radiation towards and into asphalt layer 600. An operator may
interface with gauge 200 to initialize a density measurement process in a
backscatter mode for implementation by MPC 270.

[0121] In block 1202, a data collection time of radiation detector is set
to a predetermined time period (e.g., between 15 and 30 seconds). Next,
in block 1204, energy values Ei and Ef for the energy window are
obtained. The detector counts may be communicated to MPC 270 for use in
determining density of asphalt layer 600 in a backscatter mode.

[0122] In block 1206, steps similar to the steps described with respect to
block 504-518 may be implemented for determining low window counts CL and
high window counts CH. As stated above, CL can represent the counts in a
low-energy window with low and high energy limits (ELl and
ELf), and CH can represent the counts in a high-energy window with
low and high energy limits (EHl and EHh).

[0123] In block 1208, MPC 270 may determine Rc ratio and count ratio CR.
The ratio of Rc=CL/OH may be used for aggregate identification. The ratio
CR═CH/Standard Count may be used for density determination.

[0124] In block 1210, MPC 270 may determine whether Rcis less than R0. As
stated above, Rc<R0 for limestone mixes, and that Rc>R0 for granite
mixes. If it is determined that Rc is less than R0, a limestone
calibration curve is selected (block 1212). Otherwise, if it is
determined that Rc is not less than R0, a granite calibration curve is
selected (block 1214).

[0125] In block 1216, MPC 270 may determine raw density ρ using the
limestone calibration curve. Further, in block 1218, MPC 270 may
determine void content V. MPC 270 may also select a limestone calibration
curve for surface roughness (block 1220). In block 1222, MPC 270 may
calculate a density correction dρ. In one example, d.ρ may be
determined by using one of the above equations showing the empirical
relationship between d.ρ and V. In block 1224, MPC 270 may determine
the density of asphalt layer 600 by adding raw density ρ and density
correction dρ.

[0127] Soil density measurements may be determined in a similar manner to
the asphalt density measurements. Some soils may have minerals having
high atomic number elements, such as K and Fe. According to one
embodiment, an energy-selective detector may be used for identifying soil
type based on features in the low-energy part of the spectrum. By using a
predetermined calibration for the identified soil type, density errors
may be reduced or avoided. Further, a correction to the gamma
radiation-based density measurement may be made based on a determined
moisture density. Soil density measurements may be determined using gauge
200 configured in the transmission mode shown in FIG. 3.

[0128]FIG. 13 is a flow chart illustrating an exemplary process for
density measurements in a transmission mode using gauge 200 shown in FIG.
3 according to an embodiment of the subject matter described herein.
Referring to FIG. 13, in block 1300, gauge 200 is positioned as shown in
FIG. 3 on a top surface of sample material 212, which is soil in this
example. Further, source rod 214 is positioned in a transmission mode
such that radiation source 202 is positioned in the interior of soil 212
within a vertical access hole 278 formed in soil 212. In the transmission
mode, gamma radiation emitted by radiation source 202 can directly
transverse through soil 212 to radiation detector 204. An operator may
interface with gauge 200 to initialize a density measurement process in a
transmission mode for implementation by MPC 270.

[0129] In block 1302, a data collection time of radiation detector is set
to a predetermined time period (e.g., between 15 and 30 seconds). Next,
in block 1304, energy values El and Eh for the energy window
are obtained. The detector counts may be communicated to MPC 270 for use
in determining density of soil 212 in a transmission mode.

[0130] In block 1306, steps similar to the steps described with respect to
block 504-518 may be implemented for determining low window counts CL and
high window counts CH. As stated above, CL can represent the counts in a
low-energy window with low and high energy limits (ELl and
ELf), and CH can represent the counts in a high-energy window with
low and high energy limits (EHl and EHh).

[0131] In block 1308, MPC 270 may determine Rc ratio and count ratio CR.
The ratio of Rc=CL/CH may be used for aggregate identification. The ratio
of CR=CH/Standard Count may be used for density determinations.

[0132] In block 1310, MPC 270 may identify a soil type of soil 212 based
on the value of Rc.

[0133] Based on the identified soil type, a raw density ρ of soil 212
may be determined using a calibration curve corresponding to the
identified soil type (block 1312). MPC 270 may be operable to determine
the raw density ρ using the calibration curve. The calibration curves
for various soil types may be generated based on calibration block
calibrations. As stated above, exemplary calibration blocks include Mg,
Mg/Al, and Al.

[0134] Next, in block 1314, a moisture content M of soil 212 may be
determined using moisture property detector 250. Moisture content may be
determined using a neutron-based technique or an electromagnetic-based
technique.

[0136] The calculated density value may be displayed to an operator via
display screen 274. In one embodiment, the density calculation are
carried out repeatedly at frequency intervals as measurements are made,
such as every one to two seconds. Instead of waiting until the end of a 2
to 4 minute count to display the density value, this approach makes it
possible to provide to the operator an almost real-time display of the
calculated density value while the count is still proceeding. The density
values may be displayed to the operator graphically as a function of
time. As the density value settles to a steady state, the operator may
decide to accept the calculated density value as being sufficiently
accurate, and to discontinue the measurement procedure. The radiation
source/detector and moisture property detector components may be
positioned in any suitable position in the interior or the exterior of a
gauge. For example, a moisture signal source may be positioned in an end
of a source rod for generating an electromagnetic field from within an
interior of a sample material. In this example, a moisture signal
detector may be positioned within a gauge housing for detecting the
electromagnetic field transmitted through the sample material and
generating a signal representing the detected electromagnetic field.
Further, the generated electromagnetic field may be an electromagnetic
pulse or step. In another example, a moisture signal source and detector
may be attached to a drill rod operable to penetrate a sample material
for positioning the moisture signal source in the interior of the sample
material. In this example, the moisture signal detector may generate a
signal representative of detected electromagnetic fields, and communicate
the signal via a wired or wireless communication connection to an MPC in
a gauge housing.

[0137] A moisture property detector according to the subject matter
described herein may include one or more of several electromagnetic-based
components. For example, the moisture property detector may include a
duroid patch antenna configured to detect an electromagnetic field
generated by an electromagnetic field source. The resonance frequency or
input impedance may be monitored as a function of a dielectric constant.

[0138] In another example, a moisture property detector may include a
cavity-backed dipole antenna. The antenna may include a dipole operable
at predetermined frequency (e.g., 2.45 GHz). Further, the antenna may
include a metallic cavity filled with a dielectric material to decrease
the overall size of the component. The cavity may function as an energy
focus based upon the geometry of the cavity surface.

[0139] In another example, a moisture property detector may include a
monopole. The monopole may detect broadband DC to microwave
electromagnetic fields. In use, the monopole may be driven by an
oscillator. The impedance may be measured as a function of frequency and
various soil parameters obtained. Alternatively, the impulse response can
be obtained and convolution and transform theory by be applied for
obtaining soil properties. Further, the monopole may be coated by an
insulator to reduce the energy loss in the soil.

[0140] In yet another example, a moisture property detector may include a
suitable fringing field, low-frequency device. The device may include a
signal line, a ground, and one or more conductors.

[0141] It will be understood that various details of the subject matter
described herein may be changed without departing from the scope of the
subject matter described herein. Furthermore, the foregoing description
is for the purpose of illustration only, and not for the purpose of
limitation, as the subject matter described herein is defined by the
claims as set forth hereinafter.

Patent applications by Robert Ernest Troxler, Raleigh, NC US

Patent applications by TROXLER ELECTRONIC LABORATORIES, INC.

Patent applications in class GEOLOGICAL TESTING OR IRRADIATION

Patent applications in all subclasses GEOLOGICAL TESTING OR IRRADIATION